A styrenic polymer (hereinafter abbreviated to "SPS" sometimes) having a syndiotactic configuration obtained by improving a styrenic polymer having an atactic configuration is excellent in heat resistance and chemical resistance.
The group of the present inventors has already succeeded in the development of a styrenic polymer having a high syndiotacticity, and this group has further disclosed that the SPS can be obtained by the use of a catalyst comprising a titanium compound, a catalytic product (an alkylaluminoxane) of an organic aluminum compound and a condensing agent, or a coordinated complex compound comprising a cation and an anion in which a plurality of groups are bonded to a metal (Japanese Patent Application Laid-open Nos. 187708/1987 and 249503/1992).
The above-mentioned styrenic polymer is prepared by a non-solvent polymerization method or a slurry polymerization method, but in the case that the above-mentioned styrenic polymer is prepared by such a method, it is advantageous from the viewpoints of a catalytic activity and productivity that styrene is highly concentrated and then polymerized. However, this preparation is carried out by the use of a catalyst containing a metal such as Al, Ti or Fe, and therefore a large amount of a metallic component remains in the obtained polymer, depending upon the kind of selected catalyst. This remaining metallic component causes the deterioration of the molecular weight and color tone of the polymer. Accordingly, it has been desired to decrease the remaining catalytic component in the obtained polymer product by a deashing treatment (the removal of the remaining catalyst).
As this deashing treatment method, there are a method which comprises adding a deashing agent, a method which comprises swelling the polymer with a swelling agent, and a method which comprises raising a treatment temperature in each of these methods to improve a deashing efficiency.
However, these methods have some problems. For example, the deashing efficiency is poor, washing is required, and a stable operation is difficult to do.
That is to say, in the method which comprises adding the deashing agent for the treatment, a step for removing the deashing agent is essential, and for the sake of the removal, a large amount of a solvent must be used, which makes the preparation process intricate and increases the cost of facilities. In the method which comprises adding the swelling agent for the treatment, deactivation and deashing can be accomplished under relatively mild conditions, but the polymer might dissolve or cohere, depending upon the composition and conversion of the polymer, the composition of a treatment solution and a treatment temperature. Owing to this fact, a stable operation cannot be accomplished. In addition, since the polymer swells, a waste liquid containing a remaining catalyst solution gets into the polymer, and thus the amount of the metal cannot be decreased to a certain level or less.
Therefore, it has been desired to develop a method for stably and easily preparing the SPS which can effectively inhibit yellowing and which has good characteristics.
On the other hand, in the case that the above-mentioned styrenic polymer can be prepared by a non-solvent polymerization method or a slurry polymerization method, about 0.1 to 50% by weight of a remaining volatile component such as an unreacted monomer is contained in the obtained polymer. Therefore, a drying volatilization treatment has been carried out as a post-treatment subsequent to the polymerization by a drying machine to remove a remaining volatile component such as the monomer therefrom.
However, in the case that the removal of the remaining volatile component is made by the use of the drying machine, a residence time is prolonged, which is not economical. In addition, a large amount of the unreacted monomer and the like easily remains in the product, and in this case, molding failure occurs, and the transparency of films or the like deteriorates, so that such molded articles are not desirable any more as wrapping materials for foods.
Therefore, it has been desired to develop a method for economically and efficiently removing a remaining volatile component such as an unreacted monomer from the SPS prepared by the non-solvent polymerization method or the slurry polymerization method.